Changes in nerve cell activity have been correlated with behavioral changes in a number of experimental animals. However, to prove that these neuronal changes actually cause the behavioral change has been possible only in the simplest of these systems. I have found a fairly complex behavioral change in a simple nervous system which should be amenable to physiological elucidation. If one of the twenty-one body segments of a medicinal leech is surgically isolated from the rest of the nervous system, it gradually acquires the ability to produce swimming movements over the next two weeks. Because the nervous system of the leech is accessible to microelectrode recording while the leech swims, it has been possible to describe leech swimming movements in neuronal terms. Preliminary experiments have shown that synaptic connections among identified neurons in the chronically isolated segment are different after the behavioral change has occurred. I plan to characterize all the changes which take place, show how these changes cause the behavioral change, and find the cellular mechanism of this change. In thse studies, I will utilize microelectrode recording techniques as well as intracellularly injected and extracellularly applied dyes. Hopefully, these studies will provide insight into the recovery of function after brain damage. Additionally, they may show the kinds of cellular changes that occur in other behavioral changes, such as learning and memory.